The principal focus of this renewal application is to explore the complex roles of telomeres, telomerase and p53 in tumor progression and metastases. Work from the current grant has utilized telomerase knockout mice to explore the complex roles of telomeres and telomerase in normal tissue regeneration, the aging process, acquired and inherited degenerative disorders, and cancer. Analysis of this model on the molecular, cellular and organismal levels has established that intact telomeres are essential for the long-term preservation of tissue stem cells and organ function, that telomere dysfunction is a key pathogenetic element in age-related and degenerative disorders, and that p53 is a nodal point in sensing telomere dysfunction and in executing a complex array of telomere checkpoint responses across different tissue compartments. Most relevant to the renewal application, this model also established that telomere dysfunction drives cancer initiation (particularly epithelial cancers) by provoking cancer-associated chromosomal structural aberrations, particularly amplifications and deletions. While cancer initiation is increased, the current work also suggests that ongoing telomere dysfunction and associated p53-dependent telomere checkpoint operate to constrain progression of these initiated neoplasms into advanced highly malignant disease. Upon this foundation, we now propose to explore how the telomere and p53 pathway interact to govern the survival or depletion of tissue stem cells and ultimately their transformation and evolution into fully malignant disease. Specifically, we propose the development of a novel inducible alleles for telomerase reverse transcriptase and p53 to assess the impact of somatic restoration of telomere function on stem cell depletion brought about by telomere dysfunction, to apply genetic screens to define the molecular circuitry of the telomere checkpoint response, to explore the relative contributions of telomerase reactivation and p53 inactivation in tumor progression, and to ascertain the molecular and biological response of established tumors to extinction of telomere activity and re-entry into telomere-based crisis.